please tell me why my code is not working. According to me I am doing everything correctly. question:1822E - Making Anti-Palindromes Submission :205734412

Since I want to progress, I wanted to know what all algorithms should I learn and what should I do to increase my rating.

Will N^(1.5) give TLE for N=2e5 and time is 1 seconds? In genral how many operation can be executed in 1 seconds?

I need help in the problem 1515D - Phoenix and Socks. I am getting WA on test 2, not able to figure out where I am I going wrong. Help will be appreciated. Here is my submission 189507233